1. Field of the Invention
This invention relates to sharp focus detecting devices of the TTL type incorporated in photographic instruments such as single lens reflex cameras.
2. Description of the Prior Art
Many methods have been proposed for this type of sharp focus detecting device. Of these, the secondary imaging method is an optical system such as that shown in FIG. 1. In this optical system as applied to the single lens reflex camera, a field lens 2 is positioned near or at a prescribed focal plane A of a photographic objective lens 1. At the rear of the field lens 2 is positioned an image re-forming optical system 3 comprising two image forming lenses 3a and 3b, symmetrically arranged with respect to an optical axis B of the photographic lens 1. The image is shifted in a direction parallel to the optical axis B of the photographic lens 1 and the shift in direction is perpendicular to the optical axis B. The degree of vertical shift is sensed by a sensor 4 comprising two arrays of photo-electric transducing elements 4a and 4b positioned at the rear of the image re-forming optical system 3 respectively corresponding with the image forming lenses 3a and 3b. The use of this secondary imaging method makes it possible not only to detect when in-focus, but also, when out of focus, indicates the direction in which the sharp image shifts from the focal plane A, namely, near focus or far focus, even when the degree of image sharpness is easily measured.
However, the secondary image type sharp focus detecting device has drawbacks, which because the image reforming optical system 3 and the sensor 4 must be located behind the focal plane A of the photographic lens 1, the optical system becomes relatively large in size and is difficult to build into the photographic instrument. Furthermore, the optical system must be designed so that the light from the photographic lens 1 is efficiently conducted to the image re-forming optical system 3, and even when the diaphragm of the photographic lens 1 is stopped down to a minimal aperture opening, sufficient light enters the image re-forming optical system 3.
One solution for these problems is to construct the image re-forming optical system 3 of FIG. 1, in a form shown in FIG. 2, so that the two ordinary round lenses 3a and 3b are each partly cut off at a chord C to lens fragments 5a and 5b, which are then in contact at their chords C to make up a lens unit 5. The field lens 2 of FIG. 1 has a refractive power such that an image of a circle D with its diameter equal to the length of the chord C is formed near an exit pupil of the photographic lens 1 and within the diameter of the exit pupil. It is to be noted that in this case it is, of course, possible to make up the image reforming optical system 3 with each contacting lens 5 composed of a plurality of the lens elements or fragments 5a and 5b.
By the use of the contacted lens 5 as shown in FIG. 2, the size of the optical system is minimized, and it is possible to efficiently conduct the light bundle from the photographic lens 1 to the image re-forming optical system 3. But even using the contacted lens 5 there are problems. That is, the contacted portion C of the contacted lens 5 is discontinuous in curvature. When their contacted surfaces are fixed to each other by an adhesive agent, due to facts, such as the presence of an adhesive agent layer at contacted surfaces 6, light rays L1, L2, L3, L4 incident near the interface tend to be scattered, forming ghost and flare. Also, when the lens unit 5 is formed by molding techniques, the curvature near the chord C is likely to deform, very likely producing large aberrations when focusing the images. Furthermore, as illustrated in FIG. 3, when the image re-forming optical system 3 is constructed with two or more twin-lens units 5, it is also possible that, when the light rays L1, L2, L3 and L4 enter one image forming lens 3a, one ray L4 will migrate into the opposite image forming lens 3b through the contacted surfaces 6 or air separations 7 resulting in a mixture of the light rays L1, L2, L3, L4. If this takes place, isolation and symmetry of the two images by the image re-forming system 3 is imperfect, causing deterioration of focus detecting accuracy and leading to faulty operation.
The present invention solves the above problems and its object is to provide a sharp focus detecting device with a highly sensitive degree of image sharpness. There is no possibility of faulty operation because of the improvement in the isolation and symmetry of a plurality of images formed on photo-electric transducer element arrays.
Other objects of the invention will become apparent from the following description of embodiments thereof.